Dynamic selection by a mobile station of its home agent using its preferred roaming list (PRL)

ABSTRACT

Methods and systems are provided for dynamic selection by a mobile station of its home agent using its preferred roaming list (PRL). A mobile station maintains a PRL comprising an association between (i) a first set of data identifying a first wireless wide area network (WWAN) and (ii) a first identifier of a first home agent. The mobile station detects a broadcast of the first set of data by the first WWAN. Responsive to detecting the broadcast of the first set of data by the first WWAN, the mobile station connects to the first WWAN. Responsive to connecting to the first WWAN, the mobile station uses the first identifier to register with the first home agent.

BACKGROUND

1. Technical Field

The present invention relates to wireless data communication and, moreparticularly, to improving packet-data communication for mobile stationsvia wireless wide area networks.

2. Description of Related Art

More people than ever are using mobile stations, such as cell phones andpersonal digital assistants (PDAs), to connect to wireless wide areanetworks (WWANs), which are also referred to as wireless communicationsystems, cellular communication systems, wireless networks, radio accessnetworks, wireless access networks, and by other names. Serviceproviders typically operate these WWANs to provide both voice and dataservices using a wireless communication format such as Code DivisionMultiple Access (CDMA), or another format.

Mobile stations (such as cellular telephones) typically store a“preferred roaming list” (PRL), which includes a prioritized listing ofcommunication systems (such as WWANs) that the mobile station mayaccess. In a CDMA system for instance, a PRL typically includes (i) an“acquisition table” that instructs the mobile station as to which radiofrequency (RF) channels to search and (ii) a “system table” thatspecifies, for each allowed system, a system identification code (SID)and a network identification code (NID). Each CDMA base stationbroadcasts its SID and NID in system overhead messages. Thus, when amobile station detects an allowed SID-and-NID combination, the mobilestation may connect to the identified system.

In practice, an initial or default PRL is typically loaded into a mobilestation before a wireless carrier distributes the mobile station to asubscriber. The carrier may thereafter load a new PRL into the mobilestation using a physical connection (perhaps at a customer servicecenter) or perhaps by using over-the-air service provisioning (OTASP),relevant aspects of which are described in “Over-the-Air ServiceProvisioning of Mobile Stations in Spread Spectrum Systems,” EIA/TIAIS-683-C (Mar. 20, 2003), which is incorporated herein by reference. Inoperation, mobile stations connect with various WWANs listed on theirPRLs according to the listed priorities, and perhaps according to otherlogic.

As mentioned above, mobile stations connect to WWANs to engage in bothvoice and packet-data communications. The packet-data communications maytake the form of, as examples, web-browsing communications, e-mailcommunications, Voice over Internet Protocol (VoIP) communications,and/or any other type of packet-data communications. To engage in thesepacket-data communications, mobile stations may use a packet-switchedprotocol such as the Internet Protocol (IP), relevant aspects of whichare described in “Internet Protocol,” RFC 791 (September 1981), which isincorporated herein by reference.

Furthermore, to handle their own mobility, some mobile stations may usea mobility protocol known as “Mobile IP,” relevant aspects of which aredescribed in “IP Mobility Support for IPv4,” RFC 3344 (August 2002),which is incorporated herein by reference. Mobile IP is essentially away of constructing and routing IP packets to allow devices to changetheir point of attachment to the Internet while maintaining an at leastsemi-permanent IP address, known as a “home address,” to which othernodes may address packets.

In Mobile IP, mobile stations have a particular network known as a “homenetwork.” When the mobile station is attached to its home network, itmay receive packet data addressed to its home address on that network.When the mobile station is attached to another network, known as a“foreign network,” the mobile station acquires a “care-of address.” Thisaddress may be either an address assigned to the mobile station on theforeign network (“co-located care-of address”), or an address of adevice on the foreign network known as a “foreign agent,” through whichthe mobile station conducts communications (“foreign-agent care-ofaddress”).

Either way, the mobile station registers the care-of address with adevice on its home network known as a “home agent.” When other nodes onthe Internet address packets to the mobile station's home address, thosepackets will be routed to the mobile station's home network. There, thehome agent intercepts the packets and uses encapsulation to tunnel thepackets to the mobile station's registered care-of address. In the caseof a co-located care-of address, the packets will arrive at the mobilestation, which will then decapsulate the packets. In the case of aforeign-agent care-of address, the packets will arrive at the foreignagent, which will decapsulate the packets and then forward them to themobile station.

Thus, packets sent to the mobile station from other nodes on theInternet are routed to the mobile station's home agent, and then to themobile station. Packets sent from the mobile station to other nodes mayor may not traverse the mobile station's home agent. If “triangularrouting” is used, the mobile station addresses the outgoing packetsdirectly to those other nodes. If “reverse tunneling” is used, themobile station encapsulates packets that are addressed to the othernodes inside packets addressed to the home agent. Those packets arereceived by the home agent, decapsulated, and then forwarded to theother nodes.

In general, the greater the distance and number of nodes a packet musttraverse, the more time it will take the packet to arrive at itsdestination. Conversely, the smaller the distance and number of nodes apacket must traverse, the less time it will take the packet to arrive atits destination. Thus, in the context of Mobile IP, the choice of amobile station's home agent could impact the amount of time that packetssent to (and perhaps by) the mobile node take to reach theirdestination. Furthermore, this choice may also impact the amount oftraffic a given network or group of networks is tasked with handling:the longer the packets take to reach their destination, the larger thenumber of packets that will be traveling at any given time.

SUMMARY

Methods and systems are provided for dynamic selection by a mobilestation of its home agent using its PRL. In one embodiment, the presentinvention may take the form of a method. In accordance with the method,a mobile station maintains a PRL that includes an association between(i) a first set of data identifying a first WWAN and (ii) a firstidentifier of a first home agent. The mobile station detects a broadcastof the first set of data by the first WWAN. Responsive to detecting thebroadcast of the first set of data by the first WWAN, the mobile stationconnects to the first WWAN. Responsive to connecting to the first WWAN,the mobile station uses the first identifier to register with the firsthome agent.

These as well as other aspects and advantages will become apparent tothose of ordinary skill in the art by reading the following detaileddescription, with reference where appropriate to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Various exemplary embodiments are described herein with reference to thefollowing drawings, wherein like numerals denote like entities.

FIG. 1 is a simplified block diagram of a communication system that maybe used in accordance with exemplary embodiments;

FIG. 2 is a simplified block diagram of an exemplary mobile station thatmay be used in accordance with exemplary embodiments;

FIG. 3 is a simplified block diagram of an exemplary preferred roaminglist that may be used in accordance with exemplary embodiments; and

FIG. 4 is a flowchart of a first exemplary method that makes use of thecommunication system of FIG. 1, in accordance with exemplaryembodiments.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 1. Overview

As presently contemplated, a mobile station maintains a PRL thatincludes an association between a first WWAN and a first home agent. Inaddition, the PRL may include associations between additional WWANs andadditional home agents. When the mobile station connects to the firstWWAN, the mobile station may then, with reference to its PRL, conduct aMobile-IP registration with the first home agent. Advantageously, thismay allow the mobile station to engage in Mobile-IP communication usinga conveniently-located home agent. Also, because the mobile station'sPRL may be updated from time to time using, for example, an over-the-airprocess, a wireless carrier may periodically change the associationsbetween WWANs and home agents used by the mobile station.

The PRL may contain specific data identifying the first WWAN. Forexample, in CDMA systems, this data may take the form of a SystemIdentification Code (SID) and a Network Identification Code (NID).Furthermore, the PRL may contain a number of RF channels on which themobile station should search for SID-NID combinations. Thus, the mobilestation may scan on one of those channels, locate the SID-NID pair thatidentifies the first WWAN, and responsively connect to the first WWANthat is broadcasting this SID-NID information.

The mobile station may then use an association in its PRL between thatSID-NID pair and an identifier of the first home agent. That identifiercould be a hostname, an IP address, or some other unique identifier. Inthe case of the identifier being a hostname, the mobile station (oranother entity on behalf of the mobile station) may perform a DomainName System (DNS) query to identify an IP address. Either way, once themobile station has the IP address of the first home agent, the mobilestation may engage in a Mobile-IP registration with that home agent, andthereafter engage in Mobile-IP data communications using that homeagent.

In certain circumstances, at the time the mobile station connects to thefirst WWAN as described above, the mobile station may already beregistered with another home agent and engaged in a data session usingthat other home agent. To maintain data connectivity and allow that datasession to end normally, the mobile station may wait to register withthe first home agent until that data session has completed. In fact, themobile station may conduct a Mobile-IP re-registration with that otherhome agent via the first WWAN. Following the data session, the mobilestation may then conduct a Mobile-IP registration with the first homeagent.

2. Exemplary Architecture

a. Exemplary Communication System

FIG. 1 is a simplified block diagram of a communication system that maybe used in accordance with exemplary embodiments. It should beunderstood that this and other arrangements described herein are setforth only as examples. Those skilled in the art will appreciate thatother arrangements and elements (e.g., machines, interfaces, functions,orders, and groupings of functions, etc.) can be used instead, and thatsome elements may be omitted altogether. Further, many of the elementsdescribed herein are functional entities that may be implemented asdiscrete or distributed components or in conjunction with othercomponents, and in any suitable combination and location. Variousfunctions described herein as being performed by one or more entitiesmay be carried out by hardware, firmware, and/or software. Variousfunctions may be carried out by a processor executing instructionsstored in memory.

As shown in FIG. 1, the communication system 100 includes a mobilestation 102, RF air interfaces 104 and 106, WWANs 111 and 112, and homeagents 121 and 122. It should be understood that any number of otherentities could be present as well. For example, there could be anynumber of mobile stations and other devices in communication with WWAN111. Furthermore, there could be any number of routers, other devices,and/or networks making up all or part of any of the communication links,such as the link between WWAN 111 and HA 121.

Mobile station 102 may be any mobile device capable of communicatingwith one or more WWANs, such as WWAN 111, and of carrying out themobile-station functions described herein. As examples, mobile station102 could be a cellular telephone, PDA, laptop computer, or perhaps amulti-mode device, capable of communicating with WWANs and wirelesslocal area networks (WLANs). Mobile station 102 may communicate withWWAN 111 over RF air interface 104 using a protocol such as CDMA. Inparticular, mobile station 102 may use a protocol known as cdma2000®,described in “CDMA 2000® Series,” TIA/EIA/IS-2000 Series, Rev. A (March2000), which is incorporated herein by reference.

WWANs 111 and 112 may each be any type of WWAN capable of communicating(i) over an RF air interface with one or more mobile stations and (ii)with one or more other devices such as one or more home agents. WWANs111 and 112 may be communicatively linked with each other, as shown inFIG. 1. With regard to internal structure, each of WWANs 111 and 112 mayinclude one or more of the following components, which are known in theart, as are methods of interconnecting them: base transceiver station(BTS), base station controller (BSC), mobile switching center (MSC),packet data serving node (PDSN), circuit-switched network,packet-switched network, and/or any other components necessary to enablevoice and packet-data communication with one or more mobile stations, aswell as circuit-switched and/or packet-switched communications with oneor more other entities or networks. As examples, the WWANs may beconnected to the Internet and the Public Switched Telephone Network(PSTN).

Home agents 121 and 122 may be any network devices such as servers,arranged to carry out the home-agent functions described herein.Mobile-IP home agents are known in the art, and thus are not describedin detail here.

FIG. 1 also includes an arrow 108 extending from a solid representationof mobile station 102 to a dashed representation of mobile station 102.Furthermore, RF air interface 104 is depicted in dashed form, while RFair interface 106 is depicted in solid form. The arrow 108, the dashedmobile station 102, and the dashed RF air interface 104 are meant torepresent movement by mobile station 102 from a first situation to asecond situation. The first situation is mobile station 102communicating over RF air interface 106 with WWAN 112, which isassociated with home agent 122. The second situation is mobile station102 communicating over RF air interface 104 with WWAN 111, which isassociated with home agent 121.

b. Exemplary Mobile Station

i. General Structure

FIG. 2 is a simplified block diagram of an exemplary mobile station thatmay be used in accordance with exemplary embodiments. In particular,FIG. 2 shows that mobile station 102 includes a wireless-communicationinterface 202, a processor 204, and data storage 206, all of which maybe communicatively linked by a system bus 208.

Wireless-communication interface 202 may be any combination of hardwareand software that mobile station 102 uses to communicate over an RF airinterface with a WWAN. As such, wireless-communication interface 202 maycomprise a chipset suitable for CDMA communication, and perhaps anantenna.

Processor 204 may comprise multiple (e.g., parallel) processors, such asa general purpose microprocessor and/or a discrete digital signalprocessor. Data storage 206 may take various forms, in one or moreparts, such as a non-volatile storage block and/or a removable storagemedium. Data storage 206 may store program instructions 210,mobile-station data 212, communication protocols 214, and devicemanagement logic 216. Program instructions 210 may be executable byprocessor 204 to carry out various mobile-station functions describedherein. Mobile-station data 212 may include a PRL, and may also includeany other types of data.

Communication protocols 214 may be useful to receive data from and senddata to one or more network entities, and may include any protocolsmentioned herein, any proprietary protocols, and/or any other protocols.Compatible protocols may be stored in other entities in communicationwith mobile station 102. Device management logic 216 may be used tomanage aspects of mobile station 102 such as memory and file management.

ii. Exemplary Preferred Roaming List

FIG. 3 is a simplified block diagram of an exemplary preferred roaminglist that may be used in accordance with exemplary embodiments. Inparticular, FIG. 3 illustrates a PRL 300 that may be stored asmobile-station data 212. PRL 300 comprises an acquisition table 302 anda system table 304. It should be understood, however, that PRL 300 couldtake other forms, and store other types of data as well; furthermore,the various parts of PRL 300 could be stored in a single storage mediumor among separate storage media.

In this example, acquisition table 302 has five rows, each containing anindex for that particular row and an RF frequency on which mobilestation 102 may search for a WWAN to which to connect. As shown in FIG.3, row 310 contains the index “1” and an RF channel represented as“WWAN_111,” indicating this is an RF frequency on which WWAN 111 of FIG.1 operates. Note that “WWAN_111” is a representation, and that theentries in acquisition table 302 may contain numerical representationsof RF channels.

Row 312 contains an index “2” and an RF frequency “WWAN_112,” on whichWWAN 112 operates. In this example, WWANs 111 and 112 are CDMA networks,each operating on at least one particular frequency or CDMA channel. Assuch, the frequencies listed in acquisition table 302 for thoserespective CDMA networks would aid mobile station 102 in detecting andconnecting to those particular networks.

Rows 314, 316, and 318 respectively contain an index “3” paired with afrequency for a WWAN designated WWAN 113, an index “4” paired with afrequency for a WWAN designated WWAN 114, and an index “5” paired with afrequency for a WWAN designated WWAN 115. The reader should bear in mindthat an actual acquisition table in an actual mobile station could havemore or less entries than are illustrated in acquisition table 302.

Depicted below acquisition table 302 is system table 304. Note that theindexes listed in rows 310-318 of acquisition table 302 respectivelymatch the indexes listed in rows 320-328 of system table 304, and thateach row of system table 304 contains system-identifying data(“SYSTEM”), a preference indicator (“PREF”), an identifier for aparticular home agent (“HA”), and the aforementioned indexes (“INDEX”).Other data may also be included in a system table.

With respect to rows 320 and 322, system-identifying data is given forWWAN 111 and WLAN 112, respectively. The matching index “1” in rows 310and 320 indicates that those rows pertain to WWAN 111, while thematching index “2” in rows 312 and 322 indicate that those rows pertainto WWAN 112. Rows 314 and 324, rows 316 and 326, as well as rows 318 and328 pertain to WWANs that are not depicted in FIG. 1.

With respect to the values listed in system table 304, in addition tothe “INDEX” values, which serve to coordinate between the two tables,the “SYSTEM” column contains SID-NID pairs as described above, each ofwhich identify a particular WWAN. Every CDMA system is uniquelyidentified by the combination of SID, NID, and radio frequency. It canbe appreciated that, taken together, acquisition table 302 and systemtable 304 provide this combination for WWAN 111 and WWAN 112. CDMA basestations typically broadcast their respective SID and NID in systemoverhead messages. Thus, once mobile station 102 detects a WWAN listedon its PRL by searching the frequency listed in acquisition table 302,the mobile station can verify the identity of the network by matchingthe SID and NID broadcast on that frequency to the SID and NID stored inits system table 304.

The “PREF” column may indicate that mobile station 102 should prefercertain WWANs over others. Furthermore, it may be the case that a lowernumber in this column is associated with a greater preference. Forexample, given a choice between WWAN 111 and WWAN 112, mobile station102 may opt for WWAN 111 based on the “PREF” column.

Moreover, the “HA” column contains identifiers for particular homeagents. These identifiers could be hostnames, IP addresses, or any otherdata values (or sets of data values) that uniquely identify a homeagent. In this example, these identifiers are represented as HA_121,HA_122, etc. Thus, row 320 contains an identifier for home agent 121 ofFIG. 1, while row 322 contains an identifier 322 for home agent 122.

Thus, in this example, PRL 300 of mobile station 102 contains entriesfor five WWANs that mobile station 102 may access. Namely, mobilestation 102 may access WWANs 111 through 115. PRL 300 further includesan identifier for a home agent associated with each of these WWANs. Notethat any particular home agent could be associated with more than oneWWAN. Furthermore, PRL 300 could also contain information pertaining towhether a roaming indication should be displayed by mobile station 102while it is connected to each respective WWAN. PRL 300 may also containa list of WWANs that mobile station 102 is prohibited from accessing, aswell as other types of data, such as geographical data regarding theWWANs.

3. Exemplary Operation

FIG. 4 is a flowchart of a first exemplary method that makes use of thecommunication system of FIG. 1, in accordance with exemplaryembodiments. As shown in FIG. 4, the method 400 begins at step 402, whenmobile station 102 maintains a PRL that includes an association between(i) a first set of data identifying WWAN 111 and (ii) a first identifierof home agent 121. At step 404, mobile station 102 detects a broadcastof the first set of data by WWAN 111. At step 406, responsive todetecting the broadcast of the first set of data by WWAN 111, mobilestation 102 connects to WWAN 111. At step 408, responsive to connectingto WWAN 111, mobile station 102 uses the first identifier to registerwith home agent 121. These steps are further explained in the followingsubsections.

a. Maintain a PRL Associating a WWAN with a Home Agent

At step 402, mobile station 102 maintains a PRL that includes anassociation between (i) a first set of data identifying WWAN 111 and(ii) a first identifier of home agent 121. As an example, the mobilestation may store PRL 300, which includes acquisition table 302 andsystem table 304. Acquisition table 302 includes an index (“1”)associated with an RF channel (“WWAN 111”). Furthermore, system table304 includes the same index associated with both the first set of data(“SIDNID_1”) and the first identifier (“HA_121”). In this example, thefirst set of data therefore includes both a SID and a NID, but couldtake other forms. Also, the first identifier may be a hostname, an IPaddress, or some other identifier.

b. Detect a Broadcast by the WWAN

At step 404, mobile station 102 detects a broadcast of the first set ofdata by WWAN 111. Prior to step 404, mobile station 102 may scan one ormore of the RF channels on acquisition table 302 for data identifyingone or more WWANs. Mobile station 102 may detect that WWAN 111 isbroadcasting its SID and NID (“SID_NID_1”) on an RF channel(“WWAN_111”).

c. Connect to the WWAN

At step 406, responsive to detecting the broadcast of the first set ofdata by WWAN 111, mobile station 102 connects to WWAN 111. Step 406 maytake the form of mobile station registering for service with WWAN 111,in a manner known in the art. Note that step 406 refers to registrationfor service on a wireless network, not to registration with a homeagent.

d. Register with the Home Agent

At step 408, responsive to connecting to WWAN 111, mobile station 102uses the first identifier to register with home agent 121. Home agent121 may be a Mobile-IP home agent, and mobile station 102 may carry outstep 408 by engaging in a Mobile-IP registration with the home agent. Assuch, step 408 may involve mobile station 102 sending a registrationrequest, perhaps via a foreign agent, to home agent 121. This requestcould include an IP address of the foreign agent as the mobile station'scare-of address. The home agent may store an association between a homeaddress of mobile station 102 and that care-of address.

Step 408 may further involve home agent 121 sending a registration replyto mobile station 102 via the foreign agent. Upon receipt of theregistration reply, the foreign agent may store an association betweenthe home address of mobile station 102 and information—such aspoint-to-point protocol (“PPP”) information—that the foreign agent mayuse to communicate with mobile station 102. The foreign agent may thenforward the reply to mobile station 102. Mobile station 102 may thenengage in Mobile-IP communication using the foreign agent.

At the time mobile station 102 connects to WWAN 111, mobile station 102may be (i) already engaged in a data session via home agent 122 and (ii)registered with home agent 122. This session may have started whenmobile station 102 was connected to WWAN 112. In this situation, mobilestation 102 may wait until after that session to register with homeagent 121. In fact, to continue that session, mobile station 102 mayconduct a Mobile-IP re-registration with home agent 122 via WWAN 111, toinform home agent 122 of the new care-of address of mobile station 102.As described above, PRL 300 may include an association between (i) a setof data identifying WWAN 112 and (ii) an identifier of home agent 122,as shown in rows 312 and 322.

4. Conclusion

Although the present invention has been described in the context ofMobile IP, it could also be applied in the context of Simple IP. In thatcase, the home agents would not be Mobile-IP home agents, but rathersimple network access servers (such as PDSNs) that provide packet-dataservice to mobile stations. In this sense, the registration of step 408would take the form of a mobile station requesting and receiving an IPaddress from the “home agent.” The same types of advantages could beachieved in this context, in that mobile stations would select a networkaccess server through which to communicate with reference to their PRL,and the network access servers could be located at a point on thenetwork that would be convenient and efficient.

Various exemplary embodiments have been described above. Those skilledin the art will understand, however, that changes and modifications maybe made to those examples without departing from the scope of theclaims.

1. A method comprising: a mobile station maintaining a preferred roaminglist (PRL), the PRL comprising (1) a first association between (i) afirst set of data identifying a first wireless wide area network (WWAN)and (ii) a first identifier of a first home agent, wherein the firstidentifier comprises at least one of a hostname and an Internet Protocol(IP) address, and (2) a second association between (i) a second set ofdata identifying a second WWAN and (ii) a second identifier of a secondhome agent; the mobile station detecting a broadcast of the first set ofdata by the first WWAN; responsive to detecting the broadcast of thefirst set of data by the first WWAN, the mobile station connecting tothe first WWAN; and responsive to connecting to the first WWAN, and withreference to the PRL, the mobile station using the first identifier toregister with the first home agent.
 2. The method of claim 1, whereinthe PRL comprises an acquisition table and a system table, wherein theacquisition table comprises an index associated with a radio-frequency(RF) channel, and wherein the system table comprises the indexassociated with both the first set of data and the first identifier. 3.The method of claim 1, wherein the first set of data comprises at leastone of a system identification code (SID) and a network identificationcode (NID).
 4. The method of claim 1, wherein the first home agent is aMobile Internet Protocol (Mobile IP) home agent.
 5. The method of claim4, wherein the mobile station using the first identifier to registerwith the first home agent comprises the mobile station using the firstidentifier to conduct a Mobile-IP registration with the first homeagent.
 6. The method of claim 1, further comprising the mobile stationscanning at least one radio-frequency (RF) channel for data identifyinga WWAN.
 7. A method of claim 1, comprising: a mobile station maintaininga preferred roaming list (PRL), the PRL comprising (1) a firstassociation between (i) a first set of data identifying a first wirelesswide area network (WWAN) and (ii) a first identifier of a first homeagent and (2) a second association between (i) a second set of dataidentifying a second WWAN and (ii) a second identifier of a second homeagent; the mobile station detecting a broadcast of the first set of databy the first WWAN; responsive to detecting the broadcast of the firstset of data by the first WWAN, the mobile station connecting to thefirst WWAN; and responsive to connecting to the first WWAN and withreference to the PRL the mobile station using the first identifier toregister with the first home agent, wherein, at the time the mobilestation connects to the first WWAN, the mobile station is (i) engaged ina data session using a third home agent and (ii) registered with thethird home agent, and wherein the mobile station using the firstidentifier to register with the first home agent comprises the mobilestation waiting until after a termination of the data session to use thefirst identifier to register with the first home agent.
 8. The method ofclaim 7, further comprising the mobile station re-registering with thethird home agent via the first WWAN prior to the termination of the datasession.
 9. The method of claim 7, wherein the PRL further comprises anassociation between (i) a third set of data identifying a third WWAN and(ii) a third identifier of the third home agent, and wherein, prior tothe mobile station connecting to the first WWAN, the mobile station wasconnected to the third WWAN.
 10. A mobile station comprising: awireless-communication interface; a processor; and data storage havingstored therein (i) a preferred roaming list (PRL), the PRL comprising(1) a first association between (a) a first set of data identifying afirst wireless wide area network (WWAN) and (b) a first identifier of afirst home agent, wherein the first identifier comprises at least one ofa hostname and an Internet Protocol (IP) address, and (2) a secondassociation between (a) a second set of data identifying a second WWANand (b) a second identifier of a second home agent and (ii) instructionsexecutable by the processor to: detect a broadcast of the first set ofdata by the first WWAN; responsive to detecting the broadcast of thefirst set of data by the first WWAN, connect to the first WWAN; andresponsive to connecting to the first WWAN, and with reference to thePRL, use the first identifier to register with the first home agent. 11.The mobile station of claim 10, wherein the PRL comprises an acquisitiontable and a system table, wherein the acquisition table comprises anindex associated with a radio-frequency (RF) channel, and wherein thesystem table comprises the index associated with both the first set ofdata and the first identifier.
 12. The mobile station of claim 10,wherein the first set of data comprises at least one of a systemidentification code (SID) and a network identification code (NID). 13.The mobile station of claim 10, wherein the first home agent is a MobileInternet Protocol (Mobile IP) home agent.
 14. The mobile station ofclaim 13, wherein the instructions to use the first identifier toregister with the first home agent comprise instructions to use thefirst identifier to conduct a Mobile-IP registration with the first homeagent.
 15. The mobile station of claim 10, wherein the data storagefurther comprises instructions to scan at least one radio-frequency (RF)channel for data identifying a WWAN.
 16. A mobile station of claim 11,comprising: a wireless-communication interface; a processor; and datastorage having stored therein (i) a preferred roaming list (PRL), thePRL comprising (1) a first association between (a) a first set of dataidentifying a first wireless wide area network (WWAN) and (b) a firstidentifier of a first home agent and (2) a second association between(a) a second set of data identifying a second WWAN and (b) a secondidentifier of a second home agent and (ii) instructions executable bythe processor to: detect a broadcast of the first set of data by thefirst WWAN; responsive to detecting the broadcast of the first set ofdata by the first WWAN, connect to the first WWAN; and responsive toconnecting to the first WWAN, and with reference to the PRL use thefirst identifier to register with the first home agent, wherein, at thetime the mobile station connects to the first WWAN, the mobile stationis (i) engaged in a data session using a third home agent and (ii)registered with the third home agent, and wherein the instructions touse the first identifier to register with the first home agent compriseinstructions to wait until after a termination of the data session touse the first identifier to register with the first home agent.
 17. Themobile station of claim 16, wherein the data storage further comprisesinstructions to re-register with the third home agent via the first WWANprior to the termination of the data session.
 18. The mobile station ofclaim 16, wherein the PRL further comprises an association between (i) athird set of data identifying a third WWAN and (ii) a third identifierof the third home agent, and wherein, prior to the mobile stationconnecting to the first WWAN, the mobile station was connected to thethird WWAN.